Pop quiz: what does a sheet of two-dimensional carbon have in common with the early state of the universe? If three Spanish physicists are correct, the way graphene buckles into ripples when compressed could provide some critical insights into the Higgs boson.
Graphene is the two-dimensional version of graphite, the stuff of pencil lead. There was some doubt as to whether this was even possible — for it to be truly 2D it would have to be a mere atom thick, making it also highly unstable — but Andre Geim and cohorts at the University of Manchester in the UK succeeded in creating sheets of graphene in 2004, in perhaps the most ingenious use of scotch tape yet devised.
Scads of physicists have been investigating this new substance further ever since. Geim and Konstantin Novoselov shared the 2010 Nobel Prize in Physics for their graphene research.
Why are scientists so excited about a 2D sheet of carbon? Graphene has quantum superpowers! Much has been made of the material’s potential for creating ultrafast molecular-scale transistors, especially the fact that the electrons in graphene zip along at the speed of light, as if they had no mass — contrary to special relativity, which says no object with even the tiniest bit of mass can exactly reach the speed of light.
Other results suggest that graphene can also shoot electrons through other materials as if they were invisible, making it possible to test the so-called Klein paradox in a tabletop experiment. It’s related to quantum tunneling, in which electrons can tunnel through supposedly insurmountable energy barriers. The likelihood for tunneling decreases the higher or thicker the energy barriers that are raised, and infinitely high walls should reduce those chances to zero.
The physicist Oskar Klein, however, said back in 1929 that if particles were moving fast enough, they could pass through even infinitely high barriers. It’s the kind of bizarre behavior that would normally require superheavy atoms or black holes.
So apart from all its potential practical applications, graphene provides an interesting tabletop medium to probe some of these unusual phenomena. And now Pablo San-Jose, Francisco Guinea, and Jose Gonzalez at Madrid’s Institute for Material Science propose — in a new paper in Physical Review Letters — that their graphene model loosely mimics how the Higgs field condenses, thereby giving rise to the symmetry breaking that may have split the electroweak force into the weak and electromagnetic forces in the early stages of our universe.
Imagine a ball sitting on top of a hill. This ball can be said to be in a symmetric state because there is no distinction among various directions. But this symmetric state is inherently unstable, because at some point, the ball will spontaneously roll down the hill in one direction or another. When this occurs, physicists would say that the symmetry has been broken because the direction the ball rolled down in has now been singled out from other directions.
Cosmologists hypothesize that just after the Big Bang, the cosmos was a perfectly symmetrical universe with all four fundamental forces unified at unimaginably high temperatures. But this universe was highly unstable and at some point its symmetry was broken. So as our universe expanded and cooled, the four fundamental forces split off one by one into the separate entities we know today: gravity, electromagnetism, and the weak and strong nuclear forces.
On a less cosmic scale, some version of spontaneous symmetry breaking appears to be a crucial component in many basic physical processes, including simple phase transitions, such as the critical temperature/pressure point where water turns into ice.
It also occurs in graphene: it’s responsible for the transition of the material from flat to rippled when graphene is compressed.
And this, apparently, bears a striking resemblance to the symmetry breaking in the early universe that led to the electroweak force splitting in two. Physicists generally describe this process in terms of the Higgs field — the vibrations of which are associated with the Higgs boson — shifting from a high energy state down to its ground state. Perhaps this explanation from a recent article in Physics World will help with a handy visualization:
Not the Actual Higgs
This is heady stuff, so I promptly contacted Peter Armitage, a condensed matter physicist at Johns Hopkins University, for his take on the Spanish team’s new paper.
He thought the notion of mapping a model for “spontaneous deformations of the graphene membrane” onto a model for the Higgs mechanism seemed quite reasonable, with one caveat: he doesn’t think the model will, in the end, tell physicists anything specific about the actual Higgs boson physicists hope to find at the LHC — just provide insights into these types of mechanisms in general.
That’s because there are several different models for the Higgs mechanism.The graphene model maps nicely onto one possible model, but it might not be the model that ends up being correct. It’s definitely an example of spontaneous symmetry breaking, but, as noted previously, this happens all the time in physics, and has been observed repeatedly (e.g. in ferromagnets). There’s nothing particularly “higgs-y” about graphene compared to those other examples.
Oh well. On the upside, it’s still pretty nifty in its own right. And graphene can now join superconductors in the category of Cool Materials That Generate Mass with a Higgs-Like Mechanism in the Solid State. Yes, that’s right, a similar effect also occurs in superconductors via something called the Meissner effect. A magnetic field can only penetrate a short distance inside a superconductor, and this effect “is equivalent to saying that the photons propagating in the superconductor have acquired a mass,” according to Armitage. (Photons, remember, are usually massless particles.)
And that, for Armitage, is the most relevant aspect of the Spanish paper. “It’s incredibly interesting that nature repeats on the large scale — superconductors and graphene — and the small scale (particle physics),” he says. “The same general ideas repeat themselves in different forms.”